1. Field of the Invention
The present invention relates generally to implantable prostheses for replacing human skeletal joints, and relates more particularly to an acetabular component of an implantable orthopedic hip prosthesis.
2. Background Information
Implantable orthopedic prostheses, in one form, comprise man-made replacements for the ends and articulating surfaces of the bones of the skeleton. Such prostheses are implanted to repair or reconstruct all or part of an articulating skeletal joint that is functioning abnormally due to disease, trauma, or congenital defect. Among the various articulating skeletal joints of the human body that are eligible to be fitted with implantable orthopedic prostheses, the hip joint is often treated with such prostheses. The hip joint is a major weight bearing joint and degenerates more quickly than some other joints in the event of abnormality. Also, the hip joint plays a critical role in ambulation and quality of life, resulting in great demand for surgical correction of abnormalities.
The human hip joint involves two bones: the femur and the pelvis, each having a smooth articulation surface arranged for articulation against an adjacent articulation surface of the other bone. The femur includes at its proximal extremity a head having a convex, generally spherically contoured articulation surface. The pelvis, in pertinent part, includes an acetabulum having a concave, generally spherically contoured articulation surface. The mutually engaging articulation surfaces of the femur and the pelvis together form, functionally, a ball-and-socket joint.
One or both of the articulation surfaces of the hip joint may fail to perform properly, requiring the defective natural articulation surface to be replaced with a prosthetic articulation surface provided by an implantable prosthesis. To accommodate defects of varying scope, while permitting healthy portions of the hip joint to be conserved, a range of types of orthopedic implants is available.
The range extends from total hip prosthesis systems for replacing the articulation surfaces of both the femur and the pelvis, to less comprehensive systems for replacing only the femoral articulation surface. Commonly employed orthopedic hip prostheses include components that fall within one of three principle categories: femoral stems, femoral heads and acetabular cups. A so-called "total" hip prosthesis includes components from each of these categories. The femoral stem replaces the proximal end of the femur and includes a distal stem that is received within the medullary canal at the proximal end of the femur.
The femoral head replaces the natural head and articulating surface of the femur. The acetabular cup replaces the natural socket and articulating surface of the acetabulum of the pelvis. In some designs, the stem and head are an integral, unitary component, but more often the stem and head are separate modular components designed to be assembled together to suit the anatomical needs of the patient.
The acetabular cup component of a total hip prosthesis is configured to be received and fixed within the acetabulum of a pelvis. The pelvis is prepared to receive the acetabular cup by reaming a concavity in the acetabular bone. The acetabular cup component typically has an outer surface conforming to the concavity reamed in the acetabular bone of the pelvis, and an inner bearing cavity for receiving the head of the femoral component. The head articulates in the bearing cavity as a ball-and-socket joint to restore motion to a defective hip joint.
One known type of acetabular cup involves an acetabular shell made of a bio-compatible metal such as titanium or a titanium alloy, and a bearing insert made of a bio-compatible polymer such as ultra-high molecular weight polyethylene. The acetabular shell is shaped generally as a hemispherical cup having a dome, or apex, at a proximal end and an annular rim at a distal end. As used herein, the words proximal and distal are terms of reference that indicate a particular portion of a prosthesis component according to the relative disposition of the portion when the component is implanted. "Proximal" indicates that portion of a component nearest the torso, whereas "distal" indicates that portion of the component farthest from the torso. Between the dome and rim, the acetabular shell comprises a shell wall defined by a generally convex proximal surface and a generally concave distal surface spaced from the proximal surface. The concave distal surface defines a shell cavity having an opening at the rim of the cup for receiving the bearing insert. The bearing insert has a generally convex proximal surface configured to be received and fixed within the acetabular shell in generally congruent engagement with the concave distal surface of the shell wall. The bearing insert also has a bearing cavity that opens distally for receiving the head of the femoral component. The bearing cavity is defined by a generally spherical concave bearing surface having a radius similar to that of the femoral head component. The concave bearing surface articulates against the surface of the spherical femoral head component.
Acetabular shells of the type described can be affixed to the acetabular bone by bone screws or bone cement. If bone screws are elected, the screws are driven into the bone through the screw holes before the bearing insert is placed into the shell. The shell also can be affixed by a combination of bone screws and bone cement. The acetabular shell can be provided with more screw holes than typically would be used by the implanting physician. This provides a selection of sites for placement of the bone screws, as may be dictated by the condition of the patient's pelvic bone or by the physician's preference.
Commonly, acetabular shells of the type described also include a dome hole at the apex. A typical dome hole is coaxially aligned with the axis of symmetry of the acetabular shell and extends through the shell wall from the concave distal surface to the convex proximal surface of the acetabular shell. Often, the dome hole is internally threaded or otherwise configured for receiving an instrument for holding and positioning the acetabular shell during implantation. Also, many physicians use the dome hole to obtain visual or tactile access to the reamed acetabular bone during implantation of the acetabular shell. Such access allows the physician to confirm that the acetabular shell is fully seated in engagement with the reamed bony surface of the acetabulum. As with the screw holes, for reasons explained below, it is also desirable to provide means for occluding the dome hole.
The bearing insert is usually designed to be received within the acetabular shell and may include locking tabs or other means for fixing the bearing insert into the shell in nonarticulating relative relationship. Nevertheless, a small amount of unintended relative motion is believed to occur between the bearing insert and the acetabular shell in response to the varying load borne by the acetabular cup during use. Such small relative motion, or micro-motion, may result in wear at the interface between the bearing insert and acetabular shell that generates fine polyethylene or metal debris. According to some hypotheses, such debris can migrate out of the acetabular cup and contact bone, possibly resulting in osteolysis, which ultimately can lead to bone resorption and possible loosening of the acetabular prosthesis.